1. Field of the Invention
The present invention relates to a membrane and electrode assembly (MEA), and more particularly, to an MEA which can produce a stable catalyst layer by maintaining a metal catalyst at an inactivated state using deionized water in forming the catalyst layer, a production method of the same and a fuel cell employing the same.
2. Description of the Related Art
Recently, as portable electronic devices and cordless communication equipments have been rapidly developed, much attention is being paid to development of fuel cells as portable power sources, fuel cells for pollution-free automobiles and power-generation systems as clean energy sources.
A fuel cell is a new power generation system for directly converting the chemical energy of fuel gas, e.g., hydrogen or methanol, and an oxidizer, e.g., oxygen or air, into electrical energy. There are several different fuel cells: molten carbonate fuel cells operating at higher temperatures of approximately 500 to approximately 700° C.; phosphoric acid fuel cells operating at approximately 200° C.; alkaline electrolyte fuel cells and polymer electrolyte fuel cells operating at below 100° C. or at room temperature.
The polymer electrolyte fuel cell is subdivided into a proton exchange membrane fuel cells (PEMFC) using hydrogen gas and a direct methanol fuel cell (DMFC) using liquid methanol according to anode fuel. The polymer electrolyte fuel cell, which is a source of future clean energy that can replace fossil energy, has high power density and high energy conversion efficiency. Also, the polymer electrolyte fuel cell can operate at room temperature and can be made miniaturized. Thus, the polymer electrolyte fuel cell has very wide applications including zero-emission vehicles, home power generation systems, and power source for mobile communications equipment, medical appliances and military equipment.
In general, a proton exchange membrane fuel cell using hydrogen is advantageous in that it has high power density, but cautious handling of hydrogen gas is needed and there is demand for an additional facility, such as a fuel reforming apparatus for reforming methane, methanol or natural gas to produce hydrogen fuel.
On the other hand, although having lower power density than gaseous fuel cells, a direct methanol fuel cell is considered to be suitable as a small and general-purpose portable power source from the viewpoints of manageability, low operation temperatures and no necessity of additional fuel reforming apparatus.
A basic unit of a direct methanol fuel cell (DMFC), MEA, is constructed of electrodes, that is, an anode and a cathode, disposed at opposite sides of an electrolyte, for oxidation and reduction. Each of the anode and cathode includes a support plate for maintaining the shape and strength of a cell, a diffusion layer for supplying fuel into the cell and a catalyst layer for reacting the fuel to be decomposed into ions.
The DMFC operates based on the following principle.
First, a methanol solution as fuel is supplied to the anode. The fuel absorbed into carbon paper used as the support plate is evenly diffused throughout the surface of the diffusion layer through pores present in the diffusion layer and is induced into the catalyst layer. The methanol solution induced into the catalyst layer reacts in the presence of a catalyst Pt—Ru to then be decomposed according to the electrochemical reactions below:CH3OH→(CH3OH)ads(CH3OH)ads→(CH2OH)ads+H++e−(CH2OH)ads→(CHOH)ads+H++e−(CHOH)ads→(COH)ads+H++e−(CHOH)ads→(CHO)ads+H++e−(COH)ads→(CO)ads+H++e−(CHO)ads+H2O→CO2+3H++3e−(CO)ads+(H2O)ads→CO2+2H++2e−
Protons produced by sequential decomposition steps by the action of the catalyst at the anode are transferred to the cathode across an electrolyte layer. Electrons produced simultaneously with the protons are also transferred to the cathode via the diffusion layer, a current collector and an external circuit. Carbon present in methanol reacts with deionized water to then be exhausted in the form of CO2 gas.
In the cathode, oxygen present in the air is supplied through the support plate and is evenly diffused into the MEA via the gas diffusion layer. The oxygen transferred to the catalyst layer reacts with a Pt catalyst to produce oxygen ions and simultaneously reacts with the protons and electrons transferred from the anode to produce water.
In the anode of a DMFC, methanol oxidation occurs to produce protons and electrons. The produced protons and electrons are transferred to the cathode. In the cathode, the protons react with oxygen, that is, oxidation occurs. An electromotive force based on the oxidation reaction of the protons is an energy source of a fuel cell. The following reaction equations represent reactions occurring in the anode and cathode and an overall reaction occurring in the single cell.
[Anode]CH3OH+H2O→CO2+6H++6e−Ea=0.04 V
[Cathode] 3/2O2+6H++6e−→3H2O Ec=1.23 V
[Single Cell]CH3OH+ 3/2O2→CO2+2H2O Ecell=1.19 V
The overall performance of a fuel cell is greatly influenced by the performance of anode materials because the methanol decomposition in the anode determines the rate of the overall reaction. Thus, in order to realize commercialization of DMFCs, development of superb catalysts for methanol oxidation is quite important.
Known patents including U.S. Pat. Nos. 5,415,888, 4,185,131, 5,869,416 and 5,882,810 describe the use of alcohol or organic solvent in the manufacture of a catalyst layer and a diffusion layer without separate techniques or steps for preventing oxidation of a catalyst.